The present invention generally relates to a fuel supply system for an internal combustion engine and, more particularly, to a fuel supply system having a supercharger incorporated therein for boosting the power of the engine.
The use of a supercharger, or a turbo charger or turbo supercharger as it is generally called, is not a new and recent development. The supercharger is known as a means effective to boost the power of the engine with minimized fuel consumption and generally contains a compressor and a turbine linked together by a common shaft. The supercharger is usually driven by the turbine which is in turn driven by exhaust gases emitted from the engine, although on some engines the supercharger is driven by a belt or gears from the engine instead of by an exhaust-gas turbine.
There is known an automobile fuel supply system wherein the supercharger is installed at a position upstream of the primary and secondary or auxiliary throttle valves with respect to the direction of flow of an air-fuel mixture towards the engine.
In this known type of engine fuel supply system, because the amount of incoming air to be supplied to the engine in admixture with fuel can be controlled in multi-stage fashion, the use of the supercharger has brought about such advantages as improved accuracy in the metering of the incoming air, improved maneuverability of the acceleration pedal during low speed driving of an automobile and improved availability of negative pressure for both the exhaust-gas recirculating system and the ignition distributor.
However, some disadvantages are also present in the above described known system. Specifically, in order to minimize, or substantially eliminate, the possibility that lubricant oil used in the supercharger particularly for lubricating the common shaft may leak into the fuel supply system under the influence of a negative pressure developed during a low load operating condition of the engine, a limitation has been imposed that the throttle valves have to be installed at a portion of the fuel supply system downstream of the supercharger. Accordingly, in the known system wherein the secondary or auxiliary throttle valve is so designed as to be controlled by a negative pressure developed inside a venturi area which is formed inside a fuel passage at a position upstream of the primary throttle valve, the negative pressure inside the venturi area tends to be counterbalanced with a positive pressure supplied from the supercharger during a high load operating condition of the engine and, therefore, the secondary throttle valve cannot be controlled properly. In order to avoid this problem, it is possible to control the secondary throttle valve by the utilization of a positive pressure inside a fuel supply passage at a position upstream of the primary throttle valve, this possibility, however, bringing about another problem that an abrupt increase of the positive pressure which would instantaneously occur upstream of the primary throttle valve during deceleration of the engine leads to reduction in responsivity of the secondary throttle valve to return to a proper position, thereby resulting in an impairment of the feeling of deceleration.
In order to solve this problem, a fairly recent version of the automobile fuel supply system has been so designed that, while the primary throttle valve is operatively coupled to the acceleration pedal, the secondary throttle valve is operatively linked with the primary throttle valve by means of a mechanical linkage so designed as to forcibly open the secondary throttle valve when the acceleration pedal has been depressed to open the primary throttle valve to a predetermined opening, for example, 40.degree..
In this recent version, since the secondary throttle valve is forcibly opened irrespective of the amount of incoming air when and after the primary throttle valve has been opened to the predetermined opening, the secondary throttle valve is unnecessarily opened even during the particular engine operating condition, for example, low-speed operating condition and high load operating condition, in which the supply of the incoming air controlled only by the primary throttle valve is considered sufficient.